I've not actually tried librewolf, so I can't give an exhaustive list of differences, but one of the main ones is going to be the UI. Zen has it's own UI that's specific to zen and made to be very customizable. Also the librewolf site said ublock is already installed, but isn't on zen, so you would have to set that up on your own. Plus it looks like the security features for zen are the same as the default Firefox security features, whereas librewolf has some additional security features.